Resolution Deterioration of Scanning Transmission Electron Microscope in a Windowed Gas Cell.

Abstract

Commercially available windowed gas cells for in situ scanning transmission electron microscopy (STEM) opened vast possibilities to study gas-solid interactions with high-spatial resolution. Modern gas cell designs comprise efforts to maintain the high-spatial resolution of the primary electron beam by reducing SiN window and gas thickness. Despite these efforts, the primary electron beam still interacts with the pre-sample gas atmosphere and SiN window, which leads to the deterioration of the initial spatial resolution of the primary electron beam and degrades the STEM image quality. In the presented work, we aim to understand the STEM resolution deterioration mechanisms by utilizing Monte Carlo simulations to reveal information on electron scattering in the SiN window and pre-sample gas atmosphere. Additionally, we use the derived understanding of the STEM resolution deterioration mechanisms to propose measures to avoid STEM resolution deterioration in in situ gas cell STEM experiments. Monte Carlo simulations reveal that the STEM resolution limiting factor in the gas cell is an insufficient signal-to-noise ratio (SNR). By increasing the SNR in the acquired STEM images, the resolution in the STEM images is improved. The proposed approach is demonstrated on a WS₂ specimen imaged under 0 and 1000 mbar Ar gas pressure.